1. Field of the Invention
The invention relates to a safety valve for a subterranean well of the type employing a pivotally movable flapper which cooperates with an annular valve seat defined on a well conduit.
2. Description of the Prior Art
Flapper type safety valves have long been employed in subterranean wells. One of the common forms of actuating mechanisms for such flapper valve is an actuating sleeve having a piston shoulder formed thereon upon which fluid pressure is imposed to drive the sleeve downwardly and thus pivot the flapper valve from a transverse, closed position to a vertical, open position. Due to the accumulation of tolerances involved in the assemblage of the flapper valve and the actuating sleeve to the well conduit, it often happens that the actuating sleeve engages the upper surface of the flapper valve at a point that is closely proximate to the pivot mounting axis of the flapper valve. In many cases, the well pressure below the flapper valve is in excess of the fluid pressure existing above the valve so a substantial fluid pressure differential exists across the flapper valve opposing its movement. If the actuating sleeve only contacts the flapper valve at a region close to its pivotal axis, it is obvious that a substantially greater force must be applied by the sleeve to the flapper valve to effect its opening.
In addition to the high opening forces which can be created by a pressure differential from below a flapper valve, problems can also arise as the flapper valve closes under large pressure differentials. Damage can result when the flapper valve, moving rapidly under the influence of large pressure differentials, strikes the stationary valve seat. Damage to the flapper valve, to the valve seat and to the hinge pin can seriously affect the performance of a flapper type safety valve.
In prior art flapper valves, such as that shown in U.S. Pat. No. 3,375,874, protrusions extending from the outer edges of the upper surface have been used to establish initial contact between the flapper valve and a flow actuating tube at a location spaced from the hinge or pivot of the valve. These spaced protrusions have served to increase the moment acting on the flapper valve in opposition to pressure differentials below the valve. These protrusions can, however, create space problems when the valve is open and the actuating sleeve extends past the protrusions and can reduce the flow area available through the valve when a flapper valve having a flap upper surface is used.
One means of providing additional space to permit full opening of the flapper valve is to utilize an eccentric housing. For example the safety valve shown in U.S. Pat. No. 3,726,341 employs an eccentric housing used with a flapper having a laterally offset pivot or axle means.
One other means of solving this space problem is disclosed in U.S. patent application Ser. No. 280,039 filed July 6, 1981. This flapper valve configuration is equivalent to a section cut through a tubular member about an axis normal to the axis of the tubular member.
The flapper valve disclosed and claimed herein combines a structure adapted to overcome the problems arising from large pressure differentials existing below the valve and the dimensional constraints required for a valve with the largest possible flow area. The configuration of the valve and the flapper components also results in a high degree of strength for the operating mechanism so that the valve can be used in the presence of large forces.